SUBROUTINE KNBVXK(NTRK, ITRK, IVOPT, UPDATE, VTX, VZ, LCOVAR,
     *                  CHISQ, IK, IDTYP)
 
C  >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
C  Build a new particle from several particles with a vertex constraint.
C  Add the resulting track to the KN list.
C
C  Note: If there are fewer than 2 tracks with IVOPT = 2, then ZI is
C        treated as a fixed vertex and all tracks with IVOPT > 0 are
C        required to go through it.
C
C  NTRK      Integer variable (read)
C            # of input tracks
C
C  ITRK      Integer array (read)
C            List of KN tracks.
C
C  IVOPT     Integer array (read)
C            IVOPT(IL) = 0  => Track ITRK(IL) has no position info at all
C            IVOPT(IL) = 1  => Track ITRK(IL) is not used to determine vertex
C                              but is constrained to pass through vertex
C            IVOPT(IL) = 2  => Track ITRK(IL) is used to determine vertex
C
C *VTX       real array (read)
C            Initial (x,y,z) vertex (it is updated)
C
C *VZ        real array (read)
C            3x3 covariance matrix of VTX
C
C  UPDATE    integer variable (read)
C            0 ==> do not update input track parameters (just calculate chisq)
C            1 ==> update input track parameters only
C            2 ==> update input track parameters & covariance matrices and
C                  replace the original unconstrainted parameters by the
C                  new ones. ****** This is a dangerous option. ******
C
C  LCOVAR    logical variable (read)
C            TRUE  ==> compute PW covariance matrix
C            FALSE ==> do not compute PW covariance matrix
C
C *CHISQ     real variable (write)
C            Chisq of fit.
C
C  IK        integer variable (read)
C            KN track number to put resulting track
C
C  IDTYP     integer variable (read)
C            Particle type of particle
C  >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>